An image sensor is utilized to convert an optical image focused on the sensor into electrical signals. A typical image sensor includes an array of light detecting elements, where each element produces a signal corresponding to the intensity of light impinging on that element when an image is focused on the array. These signals may then be used, for example, to display a corresponding image on a monitor or otherwise used to provide information about the optical image.
Solid-state image sensors used in, for example, digital video cameras, are presently realized in a number of forms including charge coupled devices (CCDs) and CMOS image sensors. These image sensors are based on a two dimensional array of pixels. Each pixel includes a sensing element that is capable of converting a portion of an optical image into an electronic signal. These electronic signals can then be utilized to regenerate the optical image on, for example, a display.
A CMOS image sensor is thus generally a device that converts an optical image into electrical signals using MOS (Metal Oxide Semiconductor) transistors. As compared with the CCD image sensor, the CMOS image sensor may be easily driven with various scanning schemes and integrated with a signal processing circuit on one-chip. Therefore, the CMOS image sensor may be miniaturize in size and, consequently, a reduction in the fabricating cost and the power consumption may be realized using a compatible CMOS technology.
CMOS image sensors first appeared in 1967. However, CCDs have prevailed since their invention in 1970. Both solid-state imaging sensors depend on the photovoltaic response that results when silicon is exposed to light. Photons in the visible and near-IR regions of the spectrum have sufficient energy to break covalent bonds in silicon. The number of electrons released is proportional to the light intensity. Even though both technologies use the same physical properties, all-analog CCDs dominate vision applications because of their superior dynamic range, low fixed-pattern noise (FPN), and high sensitivity to light.
More recently, however, CMOS image sensors have gained in popularity. CMOS image sensors are widely utilized in videophones, digital cameras, cellular phones, aerospace applications, PC cameras and so forth. The market share of CMOS image sensors is expected to increase in the near future, along with high volume growth for such products. Currently, the most popular pixel array for CMOS image sensors is the VGA format, well known in the art. The mainstream product, however, of CMOS image sensors is expected to be Mega pixel arrays, which are presently in an early state of fabrication.
Pure CMOS image sensors have benefited from advances in CMOS technology for microprocessors and ASICs and provide several advantages over CCD imagers. Shrinking lithography, coupled with advanced signal-processing algorithms, sets the stage for sensor array, array control, and image processing on one chip produced using these well-established CMOS techniques. Shrinking lithography can also decrease image-array cost due to smaller pixels. However, pixels cannot shrink too much, or they have an insufficient light-sensitive area. Nonetheless, shrinking lithography provides reduced metal-line widths that connect transistors and buses in the array. This reduction of metal-line widths exposes more silicon to light, thereby increasing light sensitivity. CMOS image sensors also provide greater power savings, because they require fewer power-supply voltages than do CCD imagers. In addition, due to modifications to CMOS pixels, newly developed CMOS image sensors provide high-resolution, low-noise images that compare with CCD imager quality.
CMOS pixel arrays lie at the heart CMOS image sensors. CMOS pixel-array construction utilizes active or passive pixels. Active-pixel sensors (APS) include amplification circuitry in each pixel. Passive pixels use photodiodes to collect the photo charge, whereas active pixels can include either photodiode or photo gate light sensitive regions.
The first image-sensor devices used in the 1960s were passive pixel arrays. Each pixel of a passive pixel array includes a photodiode for converting photon energy to free electrons, and an access transistor for selectively connecting the photodiode to a column bus. After photo charge integration in the photodiode, an array controller turns on the access transistor. The charge stored in the photodiode transfers to the capacitance of the column bus, where a charge-integrating amplifier at the end of the bus senses the resulting voltage. The column bus voltage resets the photodiode, and the controller then turns off the access transistor. The pixel is then ready for another integration period.
A CMOS image sensor may be formed as an integrated circuit using a CMOS process. In such a CMOS type image sensor, a photodiode or phototransistor (or other suitable device) can be utilized as the light-detecting element, where the conductivity of the element corresponds to the intensity of light impinging on the element. The variable signal thus generated by the light-detecting element is generally an analog signal whose magnitude is approximately proportional (within a certain range) to the amount of light impinging on the element. It is known to form these light-detecting elements in a two-dimensional core array, which is addressable by row and column. Once a row of elements has been addressed, the analog signals from each of the light detecting elements in the row are coupled to the respective columns in the array. An analog-to-digital converter (ADC) may then be utilized to convert the analog signals on the columns to digital signals so as to provide only digital signals at the output of the image sensor chip.
Using current fabrication technology, it is well known that defects in the substrate cause leakage current between the gate electrodes of the image sensor, especially where the substrate defects are caused by plasma damage. It is therefore of key importance to produce a substrate surface that is free of damage and, more particularly, to be able to perform spacer etching without causing damage to the substrate surface. Current practice uses a single layer of dielectric above the spacer between the gate electrodes of the image sensor. With only a single layer of dielectric, it is difficult to sense and control the etch stop above the substrate. This difficulty in controlling the etching process results in substrate surface damage; this in turn results in leakage current between the gate electrodes of the CMOS image sensor device.
An additional problem can occur during the growth of field oxide. A phenomenon results that causes defects when the gate oxide is grown. This problem is generally referred to as “white pixels”. A thin layer of silicon nitride can form on the silicon surface (i.e., the pad-oxide/silicon surface interface). When the gate oxide is grown, the growth rate becomes impeded at the locations where the silicon nitride has been formed. The gate oxide is thus thinner at these locations than elsewhere, causing low-voltage breakdown of the gate oxide.
Based on the foregoing, it can be appreciated that a great deal of work thus remains in reducing white pixels in the development of CMOS image sensors. The present inventors realize that a significant cause of white spot problems (i.e., white pixels) in CMOS image sensors is excessive current leakage from light-sensitive (e.g., photodiode) regions. In particular, such white pixels are caused by the so-called “bird's beak” effect (i.e., bird's beak induced unwanted current). Excessive and unwanted current leakage can occur in regions that are subjected to excessive mechanical stress during fabrication, and to regions that are subjected to excessive electrical stress during device operation.
The “bird's beak” effect is associated with attempts to isolate semiconductor devices from one another. One of the most significant efforts in the design of electronic systems is the continuing effort to fit more and more active devices within a particular area of a semiconductor substrate. This effort involves the reduction of the minimum geometries of semiconductor devices. Additionally, a reduction in the spacing between adjacent semiconductor devices also aids in increasing the density of the active surface area of a semiconductor substrate. If semiconductor devices are positioned too close to one another on a semiconductor substrate, parasitic capacitances and current can develop which can degrade the performance of the circuit as a whole. As such, a great deal of effort has gone into designing methods and structures to electronically isolate adjacent semiconductor devices while still allowing the semiconductor devices to be positioned closely to one another.
One method of isolation that has been utilized is the local oxidation of silicon (LOCOS) technique. Using the LOCOS technique and resulting LOCOS structures, the surface of the active semiconductor substrate is oxidized between active regions of the semiconductor surface to prevent the electronic interaction of adjacent devices. The effectiveness of the LOCOS technique degrades significantly as devices become closer and closer together due to parasitic currents that can develop between adjacent devices beneath the LOCOS structures. These currents are referred to as “punch-through” currents and travel through the bulk semiconductor beneath the LOCOS structures. The formation of such LOCOS structures often results in the formation of so-called “birds beak” structures, well known in the art. It is the formation of such bird's beak structures that often results in unwanted currents, which in turn contributes to white pixel problems in CMOS image sensor devices.
The present inventors have thus concluded, based on the foregoing, that a need exists for a method and system for avoiding white pixel problems caused by unwanted current, particular current induced as a result of the “bird's beak” effect. The present inventors have solved this problem through the design and implementation of a unique “smart pixel” layout for CMOS image sensors products, which results in a reduction in white pixel without any additional costs, which is significant departure from the earlier attempts to address white pixel issues.